Lithium metal oxides of the formula LiMO2, wherein M is a transition metal, are important cathode (positive electrode) materials for rechargeable lithium and lithium-ion batteries. Examples of LiMO2 compounds include LiCoO2, LiNiO2, and LiMnO2. Presently, LiCoO2 is used in most commercial lithium and lithium-ion batteries as a cathode material.
LiMO2 compounds can have different crystal structures and phases, even within the same compound. For example, LiCoO2 synthesized at greater than 700° C. has a hexagonal layered structure analogous to α-NaFeO2. LiCoO2 synthesized at around 400° C., however, has a cubic spinel-like structure analogous to Li2Ti2O4. Both structures have essentially the same FCC (face centered cubic) closed packed arrangement for oxygen except the layered structure has a small distortion in the direction perpendicular to the layers. Additionally, the two structures differ in cation arrangement.
It has been determined that the cubic spinel-like LiCoO2 turns into hexagonal layered LiCoO2 when heated to temperatures above 700° C. Therefore, phase transformation between the two structures is possible and the layered structure is energetically favored only at high temperatures. Layered LiCoO2 also has an energetically favored tendency of changing into spinel LiCo2O4 when 50% of the lithium ions are removed from the LiCoO2 during electrochemical charging. See A. van der Ven et al., Phys, Rev. B 58, 2975 (1998); and H. Wang et al., J. Electrochem. Soc., 146, 473 (1999). The spinel-like LiCoO2 and spinel LiCo2O4 also have essentially the same atom arrangement except that lithium is at the octahedral 16c site in spinel-like LiCoO2 and at tetrahedral 8a site in spinel LiCo2O4.
The tendency of the phase transformation from hexagonal layered LiMO2 to cubic spinel-like LiMO2 is not unique to LiCoO2. Layered LiMnO2 also turns into spinel-like LiMnO2 only after a few cycles in an electrochemical cell. Although a cubic spinel-like LiNiO2 has not been experimentally observed, Li0.5NiO2 (50% delithiated LiNiO2) will indeed turn into LiNi2O4 spinel.
The electrochemical performance of LiMO2 compounds having a cubic spinel-like structure has been found to be particularly poor, especially compared to layered structures. Moreover, the mere presence of the cubic spinel-like structural phase within the layered phase or on the surface of the layered phase has also been found to be detrimental to battery performance. In particular, the presence of cubic spinel-like phases within the layered crystal structure impedes the diffusion of lithium ions during the charge and discharge cycles of the rechargeable lithium or lithium-ion battery. Furthermore, because the cubic spinel-like phase is energetically favored and only kinetic limitations prevent large scale phase transformation, the presence of localized cubic spinel-like structures can act as a seed for phase transformation to readily occur in the LiMO2 compound. Therefore, even the minor presence of cubic spinel-like phases, even at levels that cannot be detected by bulk techniques, such as powder x-ray diffraction (XRD), can cause problems in battery cycling.